Investigation of the combined influence of salinity and particle concentration on the adsorption of anionic and zwitterionic PFAS onto estuarine sediment using the RSM modelling approach

Salinity (S) and suspended particulate matter (SPM) are key factors influencing the sorption of micropollutants in estuaries, due to strong gradients in these ecosystems. Previous laboratory or field-based studies have typically investigated the impact of S or SPM separately. Thus, the combined effects of S and SPM as well as their interactions on the sorption of micropollutants such as per- and polyfluoroalkyl substances (PFAS) in estuarine environments still remain poorly understood. We initially investigated the adsorption kinetics of 11 anionic and zwitterionic PFAS onto estuarine sediment under one S/SPM combination in laboratory-controlled conditions, as well as their adsorption isotherms under two S/SPM combinations. We also determined their distribution coefficients (K d ) across 35 S/SPM combinations covering a wide range of estuarine conditions. The adsorption kinetics of PFAS could be described by a pseudo-second-order model (equilibrium time <24h). Sorption isotherms were fitted by both linear and Freundlich models; the linear sorption range was in the range 0.12–1.31 nM and K d varied between 0.6 and 55271 L/kg. Based on response surface modelling, both S and SPM were significant factors, i.e. K d was positively related to S (salting-out effect), while it was negatively related to SPM concentration (third-phase effect). SPM had a stronger effect than S for short-chain carboxylates, whereas S was the dominant factor for most other compounds. We also present, for the first time, evidence of a significant negative interaction between these two factors. This study therefore provides a new perspective to model the fate of PFAS at the land-sea interface.